1. Field of the Invention
The present invention relates to a honeycomb structure. More particularly, it relates to a honeycomb structure hardly generating ring cracks.
2. Background Art
Heretofore, an exhaust gas purifying device including a diesel particulate filter (DPF), a catalyst body and the like has been mounted in an exhaust system of an exhaust gas. The DPF is a filter for collecting a particulate matter (PM) mainly composed of soot. The catalyst body is the one for purifying harmful substances such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) included in the exhaust gas. Moreover, a honeycomb structure has been used as a catalyst loading body constituting this catalyst body, or the DPF.
As a possible cause for a failure of the exhaust gas purifying device, there is a generation of a ring-shaped crack in the honeycomb structure. Such a crack as generated in the honeycomb structure will be called “a ring crack”. As shown in FIG. 9, this ring crack is a crack (a ring crack 50) formed at a honeycomb structure 200 so as to be substantially perpendicular to an extending direction of cells 2. Moreover, the ring crack 50 is formed so as to extend in an outer peripheral direction in an outer periphery of the honeycomb structure. FIG. 9 is a perspective view schematically showing a state that the ring crack is generated at the conventional honeycomb structure.
It is considered that a general factor for this ring crack is a tensile stress occurring in a total length direction (the extending direction of the cells) of the honeycomb structure. The tensile stress is a stress occurring by a difference between a thermal expansion coefficient of a ceramic constituting the honeycomb structure and a thermal expansion coefficient of a metal can body housing this honeycomb structure. This tensile stress occurs by placing the honeycomb structure in the environment where heating and cooling are repeated. This tensile stress tends to increase, as a ratio (A/B) between a length (a total length (A)) of the honeycomb structure in the extending direction of the cells and a diameter (an outer diameter (B)) of the honeycomb structure in a cross section perpendicular to the extending direction of the cells increases. Therefore, the ring crack is easily generated as the ratio A/B increases. Here, the total length (A) is “the length” of the honeycomb structure “in the extending direction of the cells”. Moreover, the outer diameter (B) is the diameter “of the cross section” of the honeycomb structure “which is perpendicular to the extending direction of the cells”.
Therefore, as a method of suppressing the generation of the ring crack, there is known a method of decreasing the ratio A/B by increasing the outer diameter. Moreover, a honeycomb structure making the ratio A/B in a predetermined range has been suggested (see, e.g., Patent Document 1).    [Patent Document 1] JP-A-H09-299811